In 1984 the United States adopted a multipurpose knife, the M9 bayonet. The M9 was designed to be used in conjunction with the Armalite AR-15, the Colt M-16 service rifle and the Colt M- carbine weapons with a reserve role as a hand-held close-combat weapon for infantry. Developed by Mickey Finn, Buck Knives manufactured the first M9 bayonets for Finn's company Phrobis in 1987. The first contract for the US Army was for 315,600 pieces with two 60,000 unit options at military discretion. In total, Buck made about 325,000 bayonets between 1987 and 1989.

In addition to the manufacture of the Military Contract M9 Bayonets as described above, Buck was also manufactured M9 bayonets for the commercial market. The commercial bayonets were almost identical to the military bayonet but not up to military standard. The military contract ended in 1989. In 1990, having completed the military contract and having broken ties with Phrobis altogether, Buck Knives relocated their operations. It is of note that all military contract bayonets of this period will only have Phrobis Markings. All M9 Bayonets with Buck 188 marks are commercial knives. In 1991, Buck made 5,000 units for the US Marines on an open bid and Buck Knives won the contract for a division level field testing order. These bayonets were used to the 2nd Marine Division and many are in use today.

As with all weapon systems - as in the simple battlefield knife - the soldier truly decides the worth of a product. As in the case of the M9, the blade itself was found to be too thin and prone to breakage, no doubt due to numerous manufacturers and their differing standards. Many today prefer the solid M7 bayonet over the M9. In addition to its bayonet function and standard knife operation, the M9 can double as a wire cutter and saw.